LONDON (Reuters) - The financial services industry must speak with one voice and help the government negotiate to secure Britain's future trading relationship with Europe after it leaves the bloc, a former minister and top banking industry official said on Tuesday.

"There has to be a coherent and sensible business plan which unites all parts of the financial services industry," said Angela Knight, a non-executive director who became the face of British banking during the financial crisis as chief executive of the British Bankers' Association.

Instead, there are differing voices from banks and elsewhere in the sector asking for different things, said Knight, who is currently chairwoman of the UK Treasury's Office of Tax Simplification and a non-executive director at wealth management firm Brewin Dolphin (NYSE: BRW) and financial markets broker Tullett Prebon .

The heads of top banks, asset managers and market infrastructure firms sit on a new advisory committee headed by a former government minister, Shriti Vadera. A separate forum is being launched next month aimed at giving smaller financial firms a voice.

Others were also putting forward their own proposals, Knight told an industry event in London to mark 20 years of service by Britain's Crest real-time securities trading settlement system.

"If the different groups keep on going either to the government or to Europe saying 'do this' or 'do that', then all that will happen is government and Europe will say, 'well the Brits don't know what they want in financial services' and so you will get what you are given, rather than what you want," she said.

"Get that coherency together. I appreciate that means things are going to change. They are going to change," she added.

But calls for the government to secure a "single market lite" deal for the UK, whereby the country retains full access to the EU single market in return for not having full control over migration, were unrealistic, she said.

Instead, the sector should set priorities for government negotiators that fit into likely trade-offs but still allow firms in the sector to make money, Knight said.

Multi-currency, cross-border payments infrastructure for wholesale markets should be a key priority, but retail financial services less so, she said.

"Not everything is of equal priority," she said.

Knight said she had placed a five-pound bet that Britain would vote to leave the EU, winning her 11 pounds.